1. Field of the Invention
The present invention relates to multilayer films containing an oxygen barrier, and particularly to such multilayer films which are chlorine-free, and have an oxygen barrier layer comprising polymerized ethylene vinyl alcohol. The present invention also relates to processes for making such multilayer films.
2. Description of Related Art
In the packaging and delivery of certain food products, such as cheese, pork, smoked and processed meats, and particularly fresh red meat, there exists the need to protect the meat for a period of time from when it is first butchered until it is used by the consumer. In particular, the most pertinent requirement for protecting fresh red meat is long term protection against oxygen, because oxygen promotes microbial degradation in such meats.
Polyvinylidene chloride, also referred to as "PVDC" or "saran", has been used as a sheet wrapping material or film, for preventing the passage of oxygen to the product being packaged. However, the elimination of chlorine in such packaging materials is desirable for a number of reasons, including potentially adverse environmental impact from the production and disposal of such chlorine-containing packaging materials.
Ethylene vinyl alcohol, often abbreviated "EVOH", is an alternative, chlorine-free composition exhibiting effective oxygen barrier characteristics. EVOH, however, loses its oxygen barrier properties when subjected to moisture, and, as would be expected, fresh red meats and similar food products contain high amounts of moisture which in turn tend to degrade the otherwise useful oxygen barrier properties of EVOH. Accordingly, the use of EVOH in the packaging of fresh red meat preferably includes a moisture barrier between the meat and the EVOH, to prevent the moisture from affecting the oxygen barrier characteristics of the EVOH in the film.
In addition to providing an oxygen barrier in the packaging of meats, etc., it is also preferred that the packaging material, especially packaging films, have satisfactory physical strength, to resist abuse to which the packaged product is subjected. Olefin products such as polypropylene and polyethylene can provide sheet materials having excellent strength characteristics. However, these olefins do not act as barriers to oxygen, i.e., they exhibit high oxygen transmission rates.
It is also desirable that packaging films for meats, etc., are heat-shrinkable, i.e., that the films are compatible with heat shrinking processes used in the packaging process. As is known to those familiar with heat-shrinkable packaging materials, and processes for using same, the term "orientation" refers to various deformation processes, i.e., "stretching", carried out on plastic film materials during their manufacture. The viscous and elastic properties of most thermoplastic polymers enable those polymers to recover from deformation, i.e., "remember" both the nature of the orientation and the temperature at which the orientation takes place. As a result, a plastic material oriented at a specific deformation temperature (e.g., 200.degree. F.), and thereafter cooled to ambient or working temperatures, will, if later reheated to a temperature approaching the deformation temperature, have a tendency to return to its pre-oriented state at that temperature. Since the orientation is an expansion process, then the recovery will be a shrinking process. Thus, a further desirable property of an oxygen barrier packaging film is that any orientation occur at a relatively low temperature, with subsequent shrinking also occurring at that relatively low temperature.
Furthermore, packagers prefer to carry out the heat-shrink process at temperatures well below those at which red meat would start to cook. Thus, the lower the temperature at which a film can be oriented, the correspondingly lower temperature at which the film can be heat-shrunk, and the greater advantage the film offers for the packaging of fresh red meat. Additionally, the greater the degree to which a film can be oriented, the greater the degree to which the film will shrink. Furthermore, films which have a greater shrink potential have greater utility.
Therefore, the need exists for a chlorine-free, heat-shrinkable plastic film capable of maintaining oxygen barrier characteristics in the presence of reasonable amounts of moisture, capable of being oriented at an orientation ratio of greater than 9.times.1 at a temperature at or below 212.degree. F.